Natural photosynthetic systems are fairly sophisticated natural molecular assemblies which consist of aggregates of chlorophylls and carotenoids that absorb light to intiate a sequence of energy transport steps. Fabrication of artificial photosynthetic systems which can generate current upon photoexcitation with the same high efficiency of conversion as occurs in natural photosynthetic systems would be particularly advantageous. Organized artificial molecular assemblies are of potential use in the design of efficient devices for photoenergy conversion, photocurrent generation and photoswitching or for devices such as molecular wires and switches. Design of a highly efficient artificial photosynthetic system requires the efficient and reproducable organization of photon harvesting groups, electron donors and electron acceptors into a molecular assembly that facilitates the conversion of light into electrical current and chemical potential with high efficiency. While some workers have explored prototype supramolecular structures for such applications, to date many of these artificial systems have exhibit problems with their stability and efficiency of energy conversion.
Self-assembled monolayers (SAMs) are particularly attractive for fabrication of artificial photosynthetic system by constructing molecular assemblies with light harvesting groups on top of the monolayers. The light harvesting and charge separation character of the monolayers is evidenced by the generation of current upon photoexcitation of the SAM in the presence of an electron donor or electron acceptor compound. The basic idea for preparation of these devices is to attach a molecule, for example an alkanethiol, with a chromophore group placed on top of the layer, so as to be easily exposed on the surface and therefore to be able to absorb light from an external source and to start the photocurrent flow through the system. An example of this type of devices is given in the work by Imahori (H. Imahori et al., J. Phys. Chem. B 2000, 104, 1253-1260) who uses phorphyrins coupled to long chain alkanethiols to produce photocurrent. Other compounds, such as peptides have also been used because they facilitate electron transfer from the gold surface to the chromophore. Kimura, et. al. (T. Morita et al., J. Am. Chem. Soc. 2000, 122, 2850-2859) were the first to report the use of peptide SAMs for photocurrent generation. The choice of peptide backbones was based on the electrostatic field that helical peptides possess. Fox et al. (J. Am. Chem. Soc. 1997, 119, 5277-5285; J. Phys. Org. Chem., 1997, 10, 484-498; J. Am. Chem. Soc., 1996, 118, 2299-2300.) and Imanishi et al. (J. Phys. Chem. 1991, 95, 3847-3851) have shown that this electric field can favored the rate of electron transfer between donor-acceptor pairs located along the peptide helix.
A more complex system proposed in the work by Uosaki, et. al. (J. Am. Chem. Soc. 1997, 119, 8367-8368; Thin Solid Films. 1996, 284-285, 652-655; Journal of Electroanalytical Chemistry. 1997, 438, 121-126) is a SAM with a photoactive group on top and an electron relay group at the middle part of the molecule to facilitate electron transfer through the monolayer. Uosaki's group has reported many of these devices using thiol molecules containing groups such as porphyrin-ferrocene-thiol and porphyrin-mercaptoquinone systems. The presence of an electron relay group, of lower oxidation potential than the electroactive unit, favors the electron transfer from the gold surface towards the electroactive compound (cathodic photocurrent) or from the excited singlet state of the electroactive compound towards the gold surface (anodic photocurrent). The cathodic photocurrent is enhanced with more negative potentials applied to the working electrode, and the anodic with more positive potentials.
An even more sophisticated system has been designed by Imahori et. al. (J. Am. Chem. Soc. 2001, 123, 100-110; Chem. Commun. 2000, 661-662). Their systems consist of mixed monolayers with different chromophore groups on top, one of them to serve as antenna and enhance the photocurrent generation by energy transfer to the second chromophore. The first system described by Imahori has pyrene and porphyrin molecules. The photocurrent is enhanced by an energy transfer from the pyrene to the porphyrin. A more complex system contains a boron-dipyrrin molecule as an antenna complex and a molecule bearing a ferrocene, porphyrin and [60]fullerene. With this system, the photocurrent is produced from the gold towards the fullerene by using the ferrocene as electron relay group and porphyrin as the photoexcited molecule. The boron-dipyrrin molecule enhances the photocurrent by energy transfer to the porphyrin.
Besides complex organic molecules, other possibility for preparation of supramolecular devices is the use of metals inside the SAM structure. Some studies have been published related to the use of metals as complexing units between different organic ligands to produce multilayers and the subsequent characterization of these systems. Bard, et. al.(Langmuir. 1997. 13, 5602-5607) have used copper to produce photo-luminescent multilayer thin films formed by Cu (I) sandwiched between the carboxyl and thiol groups of bifunctional molecules such as 3-mercaptopropanoic acid. Mallouk et al. (Langmuir. 1991, 7, 2362-2369) have reported preparation of zirconium 1,2-ethanediylbis(phosphonate) multilayer films on gold and studied the electron transfer rates through the film by electrochemical measurements. Hatzor et al. (J. Am. Chem. Soc. 1998, 120, 13469-13477) studied self-assembled multilayers on gold formed with zirconium, cerium and titantium ion complexes with dihydroxamate ligands. Fabrication of organic-inorganic thin films has also been reported recently by Thompson, et. al.(J. Am. Chem. Soc. 2002, 124, 4796-4803) and Ulman, et. al. (Langmuir. 2002, 18, 6207-6216). These workers have shown that organometallic multilayered systems are a convenient alternative for fabrication of stable model surfaces with potential applications in photocurrent generating systems and chemical sensing. These workers have shown that multilayer films can be grown by sequential deposition of layers and theri formation can be monitored with characterization techniques such as ellipsometry and cyclic voltammetry.
Imahori et al. (H. Imahori et al., S. Chem. Commun. 2000, 661-662; H. Imahori et al., J. Phys. Chem. B 2000, 104, 1253-1260; H. Imahori et al., J. Am. Chem. Soc. 2001, 123, 100-110) have fabricated self-assembled monolayers (SAMs) on gold by depositing a covalently-linked, multicomponent molecule containing alkanethiol, ferrocene, porphyrin and C60 subunits. These SAMs have both light harvesting and charge separation character as evidenced by the generation of current upon photoexcitation of the SAM in the presence of methyl viologen. Fox et al. have investigated optical and electrochemical properties of SAMs in which pyrene chromophores are coupled to gold surfaces. (M. A. Fox et al., Langmuir 1998, 14, 816-820; R. S. Reese and M. A. Fox, Can. J. Chem. 1999, 77, 1077-1084. In separate work, Mallouk et al. (H. G. Hong and T. E. Mallouk, T. E. Langmuir 1991, 7, 2362-2369; H. C. Yang et al., J. Am. Chem. Soc. 1993, 115, 11855-11862), Bard (S. Ogawa et al, A. J. J. Phys. Chem. B 1997, 101, 5707-5711; M. Brust et al., Langmuir, 1997, 13, 5602-5607) and others have studied the formation of multilayer, multicomponent thin films on gold via non-covalent interactions between sulfur-containing ligands and metal ions. In one study, Bard et al. (Langmuir, 1997, 13, 5602-5607) reported thin films produced by the repeated sequential deposition of mercaptoalkanoic acids and Cu(II) ions. Kimura and co-workers (T. Morita et al., Bull. Chem. Soc. Jpn. 2000, 73, 1535-1540; T. Morita et al., J. Am. Chem. Soc. 2000, 122, 2850-2859) also observed photocurrent generation following excitation of a SAM on gold that consists of an alkanethiol linked covalently to a helical peptide containing a carbazole group at the terminal residue. While the photocurrent generating efficiency reported for these systems was promising, the synthetic effort involved in producing such multifunctional molecules was likely considerable.
Incorporation of molecular components for light harvesting and charge separation into artificial photosynthetic systems requires addition structures which exhibit high harvesting efficiencies and provided for straightforward synthesis of multilayer films. While many organic systems have been developed that exhibit reasonable light harvesting efficiencies and much has been learned about the energy transfer, electron transfer and charge separation processes that occur during photosynthesis, creating a system that can efficiently convert photons to electrons remains a primary objective and unrealized goal. Of primary concern is the creation of a highly organized supramolecular scaffold that optimizes light harvesting efficiency and transfer rates which are available with the protein framework found in natural photosynthetic systems.